In recent years, along with the spread of mobile electronic devices such as mobile phones and notebook-sized personal computers, development of smaller and lighter nonaqueous electrolyte secondary batteries having a high energy density has been strongly demanded. Development of high power secondary batteries as batteries for electric automobiles typified by hybrid automobiles has also been strongly demanded. The secondary batteries that meet such demands are exemplified by lithium ion secondary batteries. Lithium ion secondary batteries include a negative electrode, a positive electrode, an electrolytic solution and the like, in which a material into and from which lithium can be inserted and desorbed has been used as an active material for the negative and positive electrodes.
Research and development of the lithium ion secondary batteries have been extensively performed at present, and in particular, lithium ion secondary batteries in which a layer or a spinel type lithium metal composite oxide is used as a positive electrode material can achieve a voltage as high as 4 V; therefore, practical applications thereof as batteries having a high energy density have been accelerated.
As positive electrode materials for such lithium ion secondary batteries, lithium composite oxides such as lithium cobalt composite oxide (LiCoO2) which can be comparatively easily synthesized, lithium nickel composite oxide (LiNiO2) in which nickel less expensive than cobalt is used, lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), and lithium manganese composite oxide (LiMn2O4) in which manganese is used have been hitherto proposed.
In order to achieve favorable performances of positive electrodes (superior cycle characteristics, low resistance and high power), positive electrode materials are required to be formed with particles having a uniform and appropriate particle diameter. The grounds for such requirements are that use of a material having a large particle diameter and a small specific surface area leads to failure in reserving a sufficient area for reaction with the electrolytic solution, thereby resulting in an increase of the reaction resistance and failure in obtaining a battery having a high power, and that use of a material having a broad particle size distribution leads to lowering of the battery capacity, thereby resulting in defects such as an increase of the reaction resistance. The reason why the battery capacity is reduced is that nonuniformity of a voltage applied to the particles in the electrode causes fine particles to selectively deteriorate due to repetition of charge and discharge.
Additionally, when increase in output of batteries is intended, shortening a distance of transfer of lithium ions between the positive electrode and the negative electrode is effective. Therefore, thin positive electrode plates have been desired, and thus cathode active material particles having a small particle diameter are useful also to this end.
When further increase in output of batteries is intended, cathode active material particles having a small particle diameter and a large specific surface area are useful. For example, by reducing smoothness of surfaces of the particles, employing a porous structure for the particles themselves, or the like, the specific surface area can be increased even though the particle diameters of the particles are the same. This leads to increasing an area for reaction between the particles and the electrolytic solution and enables to enhance reactivity therebetween. Increase in output of batteries can therefore be expected.
Additionally, it is necessary to produce particles so as to have a small and uniform particle diameter and a large specific surface area also in the case of the above lithium nickel composite oxide in order to improve performances of the positive electrode material.
Patent Literature 1 discloses a lithium composite oxide in the form of particles having a particle size distribution exhibited on the particle size distribution curve thereof in which: an average particle diameter D50 which means that the particle diameter of the particles with the accumulation frequency of 50% is 3 to 15 μm; the minimum particle diameter is 0.5 μm or more; and the maximum particle diameter is 50 μm or less, and with respect to the relationship between D10 which means that the particle diameter of the particles with the accumulation frequency of 10% and D90 which means that the particle diameter of the particles with the accumulation frequency of 90%, D10/D50 is 0.60 to 0.90, whereas D10/D90 is 0.30 to 0.70. The Literature also discloses that the lithium composite oxide has high fillability, favorable charge and discharge capacity characteristics and high output characteristics, and is less likely to deteriorate even under conditions with a significant charging and discharging load; therefore, use of this lithium composite oxide can provide a lithium ion nonaqueous electrolytic solution secondary battery having excellent output characteristics with small deterioration of the cycle characteristics.
Patent Literature 2 discloses a cathode active material for a nonaqueous electrolytic solution secondary battery having at least a lithium-transition metal composite oxide with a layer structure in which the cathode active material for a nonaqueous electrolytic solution secondary battery includes the lithium-transition metal composite oxide formed with hollow particles having an outer shell portion outside and a space portion inside the outer shell portion. The Literature also discloses that the cathode active material for a nonaqueous electrolytic solution secondary battery has excellent battery characteristics such as cycle characteristics, output characteristics and thermal stability and is used suitably for lithium ion secondary batteries and the like.
Since the lithium composite oxide particles disclosed in Patent Literature 1 are particles whose minimum particle diameter is 0.5 μm or more and whose maximum particle diameter is 50 μm or less with respect to the average particle diameter of 3 to 15 μm, the lithium composite oxide particles include fine particles and coarse particles. The particle size distribution defined by the D10/D50 and D10/D90 described above therefore does not suggest a narrow range of the particle diameter distribution. Consequently, the lithium composite oxide disclosed in Patent Literature 1 does not correspond to particles having uniform particle diameters, and thus improvement of the performances of the positive electrode material is not expected even when such a lithium composite oxide is employed, indicating difficulty in obtaining a lithium ion nonaqueous electrolytic solution secondary battery having sufficient performances.
Since the cathode active material for a nonaqueous electrolytic solution secondary battery disclosed in Patent Literature 2 includes hollow particles, increase of the specific surface area can be expected comparing with solid particles; therefore, improvement in reactivity between the particles and the electrolytic solution can be expected because of the increase of the specific surface area. Patent Literature 2 however does not describe a particle diameter and a particle size distribution of the cathode active material for a nonaqueous electrolytic solution secondary battery, considering it to be equal in terms of quality to a conventional cathode active material. Therefore, there is a high possibility that selective deterioration of fine particles occurs due to nonuniformly applying a voltage in an electrode, resulting in a reduction in a battery capacity.